If analogue picture signals are processed after converting them to digital data in a system for processing picture data, can be corrected which errors occurred during transmissions, recordings and regenerations by utilizing an interleave, an error correction code and an interpolation. Moreover, in the case of using a recording medium such as a tape or a disc, there is no degradation of the picture quality even if copying is carried out many times.
On the other hand, there is also such a disadvantage that the amount of the data is increased due to the analogue digital conversion (to be called hereinafter "A/D conversion"), which results in lower transmitting speed and a large recording capacity is required in recording.
To solve the disadvantage, various picture data compressing methods have been proposed, and among which an RLC method has been widely used.
FIG. 1 is a block diagram showing the constitution of the conventional RLC method comprising a zig-zag scanning section 1, a run length count section 2, a grouping section 3 and a Huffman encoder 4.
FIG. 2 illustrates the scannings by the zig-zag scanning section 1 of FIG. 1.
The conventional RLC method is now described referring to FIGS. 1 and 2. In order to compress picture signals, first the picture signals are subjected to an A/D conversion and then the data are divided into a sub-block of 8.times.8 pixel units, a DCT being carried out on each of the sub-blocks thereafter. Then the 64 DCT coefficients K0-K63 as shown in FIG. 2 which are the result of the conversion from a time domain to a frequency domain, represent some frequency components depending on their positions. Among the DCT coefficients K0-K63, the coefficient K0 which is zero in its vertical and horizontal frequencies is called the dc coefficient, and the rest of the coefficients K1-K63 are called ac coefficients. Since the dc coefficient is important in rotation to the average values of the relevant sub-blocks, it is coded separately from the ac coefficients.
Generally, the ac coefficients of picture signal has such a nature that the higher their frequencies are, the greater the probability that their values are zero is. Particularly, in carrying out a zig-zag scanning as shown in FIG. 2, there is a high probability that 0 appears successively. From considering the fact, a coding is carried out by applying the RLC method as shown in FIG. 1, thereby compressing the picture data.
Meanwhile, among the DCT coefficients of one sub-block, the dc coefficient is coded separately by applying the 1-dimensional Huffman coding method, while the other 63 ac coefficients are subjected to zig-zag scannings as shown by the dotted lines in FIG. 2, in such a manner that 0 should appear successively numerously so as for the run length to increase, the scanning being carried out by the zig-zag scanning section 1 of FIG. 1.
The ac coefficients which have undergone the zig-zag scannings are separated into 0 coefficients and non-zero coefficients, and the coefficients in which 0 appears successively are grouped together, while codings are carried out for the grouped coefficients and the non-zero coefficients appearing next to the grouped 0 coefficients. That is, the run length count section 2 counts the number of the 0 coefficients until a non-zero coefficient appears after the inputting of the ac coefficients which have undergone the zig-zag scannings. Then, the grouping section 3 classifies the group involving the non-zero group into a separate group, based upon which the category and the lower bit are obtained.
If an instance is taken for the classification of the groups, classifications are made in the form of .+-.1, .+-.2.about..+-.3, .+-.4.about..+-.7 . . . , and the categories for them are 1,2,3, . . . .
Then the Huffman coder 4 produces and outputs ac codes based on the Huffman code table after receipt of the run lengths counted by the run length count section 2 and the categories grouped by the grouping section 3. Therefore, the lower bit for the non-zero coefficients of the grouping section 3 and the ac codes of the Huffman coder 4 can be transmitted or recorded.
The coding process described above is specified in the provision of the recommendation JPEG (on the data compressing) of the CCITT and ISO.
The RLC method is capable of coding 0 appearing between non-zero coefficients among the 63 ac coefficients but it has the problem that a large number of codes are required for the 0-15 run length and for the respective categories.
That is, since the coding becomes different for the respective non-zero coefficients depending on how many 0's exist before them, the table for defining the codes corresponding with the respective pairs of 0 coefficients and non-zero coefficients becomes large, and the average code lengths for the respective non-zero coefficients are increased. Accordingly, the coding procedure and the coding logic also become complicated. Furthermore, in the case that the successively appearing 0 are scattered with small numbers, for example, NOONOON . . . (N representing a non-zero coefficient), if a coding is carried out, the successively appearing 0, that is, the information for the run lengths are added to each of the coefficients in addition to the self information of the non-zero coefficients, which causes an increase of the total amount of the data.